Recent cloning of the human ileal bile acid transporter (IBAT) gene now offers the opportunity to assess whether specific mutations in this gene are causative in children with intractable diarrhea of infancy, children and adults with idiopathic diarrhea, and children and adults with hypocholesterolemia (serum cholesterol values that place them in the lowest decile). The hypotheses to be tested are: 1) a subset of children with a history of intractable diarrhea in infancy will have mutations in the IBAT gene which leads to bile acid malabsorption; 2) a subset of children and adults with idiopathic diarrhea will be shown to have mutations in the IBAT gene as a cause of their diarrhea; 3) an undefined fraction of a group of children, adolescents and adults who have serum cholesterol in the lowest decile will have mutations in the IBAT gene which causes reduced reabsorption of bile acids with consequent reduction in serum cholesterol.